Energy & Fuels 1995,9,439-447
439
Formation of Nitrogen Compounds from Nitrogen-Containing Rings during Oxidative Regeneration of Spent Hydroprocessing Catalysts Edward Furimsky," Michael Nielsen, and Peter Jurasek Energy Research Laboratories, Canada Centre for Mineral and Energy Technology, Natural Resources Canada, 555 Booth Street, Ottawa, Ontario, Canada KIA O G 1 Received December 22, 1994@
Commercial CoMo and NiMo catalysts in an oxidic and sulfided form and a y-alumina were deposited with pyrrole, pyridine, and quinoline. The deposited catalysts and two spent hydroprocessing catalysts were pyrolyzed and oxidized under conditions typical of regeneration of hydroprocessing catalysts. The formation of NH3 and HCN, as well as selected cases of N2O and NO, was monitored during the experiments. NH3 and HCN were formed during pyrolysis of pyrrole-deposited catalysts whereas only NH3 was formed during that of pyridine- and quinolinedeposited catalysts. For all deposited catalysts, both NH3 and HCN were formed during temperature programmed oxidation in 2% 0 2 . For spent catalysts, a small amount of N20 was formed in 2 and 4% 0 2 . For pyrrole-deposited catalysts, large yields of N2O were formed in 4% 0 2 . Under the same conditions, N2O yields for pyridine- and quinoline-deposited catalysts were very small.
Introduction A gradual decrease of catalyst activity during hydroprocessing can be offset by adjusting operating parameters such as temperature, Ha pressure, and residence time. At certain points, further adjustment of the operating parameters can be unsafe and the catalyst has to be replaced. The catalyst deposited by coke can be regenerated and returned to the operation. The review on regeneration of spent hydroprocessing catalysts indicates a good understanding of the reactions involving carbon and sulfur which are part of spent cata1ysts.l The conversion of nitrogen contained in the coke has attracted less attention. Zeuthen et a1.2 followed NO formation during temperature-programmed oxidation (TPO) of hydroprocessing catalysts exposed t o NH3 and butylamine. These authors also investigated catalysts which were coked with the ethylcarbazole or pyrene as well as spent hydroprocessing ~ a t a l y s t s .NO ~ was the only N-containing product measured during TPO of operating and powder forms of hydroprocessing catalyst^.^ Studies on pyrolysis of organic solids containing nitrogen confirm the formation of HCN, NH3, and N2 as the major N-containing gaseous product^.^,^ Very small amounts of 0 2 in oxidizing gas used for catalyst regeneration suggest that these compounds can be formed during regeneration. Also, the formation of N2O cannot be ruled out completely. Thus, the formation of NH3, HCN, NO, and N2O was confirmed during pyrolyAbstract published in Advance ACS Abstracts, April 15, 1995. (1)Furimsky, E.; Massoth, F. E. Catal. Today 1993,17, 4. (2) Zeuthen, P.; Bloom, P.; Muegge, B.; Massoth, F. E. Appl. Catal. 1991,68, 117. (3) Zeuthen, P.; Bloom, P.; Massoth, F. E. Appl. Catal. 1991,78, 265. (4) Furimsky, E.; Zaitlin, L.; Laugher, R. Fuel 1993,77, 1541. (5)Ohtsuka, Y.; Furimsky, E. Energy Fuels 1996,9,141. (6)Bassilakis, R.; Zhao, Y.; Solomon,P. R.; Serio, M. A. Energy Fuels 1993,7, 710. @
sis of several N-containing compounds in a fluidized bed.7 A systematic approach was used in the present study to explain the mechanism of pyrolysis and oxidation of pyrrole, pyridine, and quinoline under conditions which approach oxidative regeneration. At first, fresh oxidic and sulfided CoMo/Al2O3 and NiMo/AlzOa catalysts were deposited with these compounds to produce a model coke. Then the deposited catalysts and two spent catalysts from hydroprocessing operations were pyrolyzed and were also subjected t o TPO in 2 and 4% 0 2 . The evolution of gaseous products such as HCN, NH3, N2, NO, N20, CO, C02, and H2O was continuously monitored during the experiments.
Experimental Section Catalysts. Fresh catalysts included a n extrudate form of microporous CoMo/AlzOs and chesnut burr-like form of macroporous NiMo/AlzOs having the BET surface area of 210 and 140 m2/g, respectively. The sulfided form of the fresh catalysts was obtained by sulfiding in the 10 vol % HzS Hz mixture a t 400 "C for 2 h followed by cooling to room temperature in the flow of Nz. The catalysts were placed in a wire basket and suspended in the organic compounds (pyrrole, pyridine, or quinoline) a t room temperature for 20 min. To remove the excess liquid compounds, the baskets containing the catalysts were left suspended in air for 2 h. Subsequently, the catalysts were treated at 150 "C in a vaccum for 4 h. Activated y-alumina was low in alkalis and had a surface area of 175 m2/g. I t was deposited and treated in the same way. The same CoMo and NiMo catalysts were used in hydroprocessing operations, Le., the CoMo/AlzOs catalyst for hydroprocessing of a conventional distillate and the NiMo/AlzOs catalyst for hydroprocessing of an atmospheric residue. The former was used as received, whereas the spent NiMo catalyst was extracted either directly by toluene or by hexane followed by toluene. The extracted catalysts were treated under a vacuum to remove any excess solvent.
+
(7) Hamalainen, J. P.; Aho, M. J.;Tummavuori. Fuel 1994, 73,1894.
0887-0624/95/2509-0439$09.00/0Published 1995 by t h e American Chemical Society
Furimsky et al.
440 Energy & Fuels, Vol. 9, No. 3, 1995 Table 1. Analvsis of Catalvsts C N N/C CoMo pyrrole CoMo pyridine CoMo quinoline NiMo pyrrole NiMo pyridine NiMo quinoline CoMo pyrrole CoMo pyridine CoMo quinoline NiMo pyrrole NiMo pyridine NiMo quinoline Y-&o3
pyrrole pyridine quinoline CoMo NiMo" NiMob
Oxidic Catalysts 5.65 0.67 5.40 0.69 5.22 0.62 3.99 0.72 3.77 0.58 3.53 0.55 Sulfided Catalysts 7.17 1.22 0.93 6.42 6.44 0.94 5.00 1.44 0.37 2.97 0.53 4.90 5.46 0.57 5.11 0.34 5.93 0.38 Spent Catalysts 7.64 0.14 15.85 0.31 15.02 0.35
0.14 0.11 0.10
0.155 0.132 0.134 0.146 0.124 0.124 0.247
C+N
6.58 6.09 5.84 4.70 4.35 4.08
0.108
8.29 7.35 7.38 6.44 3.34 5.43
0.089 0.057 0.055
6.03 5.46 6.31
0.016 0.017 0.20
7.78 16.16 15.37
0.105
a Extracted by hexane followed by toluene. Extracted directly by toluene.
The carbon and nitrogen contents of the deposited and spent-extracted catalysts are shown in Table 1. Procedures. The thermal gravimetric analysis (TGA) was performed using Perkin Elmer TGA 7 analyzer with the Perkin Elmer 7599 Professional computer attached for data handling. About 10 mg of uncrushed catalyst was used for the analysis. The samples were heated from room temperature to 1000 "C a t the heating rate of 10 "C/min either in Nz (30 m u m i n ) or in air (30 mlimin). The fixed bed reactor used for the TPO experiments was made of quartz and had 10 mm i.d. External heating was provided by a Lindberg furnace. The catalyst particles were supported by a quartz wool plug. About 0.5 g of accurately weighed sample was used for the experiments. The temperature-programmed pyrolysis was performed in the flow of He (0.6 Umin) from room temperature to 1000 "C at the heating rate of 10 "Cimin. The same conditions were used for the TPO using either 2 or 4 vol 9% 0 2 + Nz balance. Separate experiments were performed for analysis of NO and N20. Because of the analyzer requirements, the flow of gas was maintained a t 1 limin and sample size was 1 g. Gas Analysis. HCN, NH3, CO, HzO, and COz analyses were performed using the on-line Bruel & Kjaer FTIR analyzer type 1301. The detection principle of the analyzer is based on photoacoustic absorption. The HCN and NH3 calibration was performed by manufacturer of the analyzer. Analyses were carried out in 2 min intervals. The MTI 200 gas chromatograph equipped with the thermal conductivity detector was used for the Nz analysis. NO analysis was performed using the chemiluminescent NONO, analyzer Model 10 AR. The NzO was analyzed using a n FTIR on-line analyzer. The yields of compounds and their rates of formation were calculated from their concentrations and a total flow normalized to STP conditions.
Results As the results in Table 1 show, larger amounts of compounds were adsorbed on CoMo catalysts than that on NiMo catalysts. This is partly attributed to the larger surface area of the former. For both catalysts, the adsorbed amount increased after sulfidation. For every series of the catalysts, the largest adsorption was exhibited by pyrrole whereas the adsorbed amounts of pyridine and quinoline were similar. The N/C ratios of
deposits were always lower than that of the corresponding compounds, suggesting preferential removal of nitrogen compared with carbon during catalyst pretreatment. It was indeed shown that the oxidation of carbon was exothermic whereas that of nitrogen was nearly thermally neutra1.l The differences in the amount of deposits between the catalysts and among the compounds offer a discussion on interaction of N-containing hetero rings with the catalyst surface.8 However, this topic is not objective of the present study. For spent NiMo catalysts, the direct extraction by toluene and the extraction by hexane followed by toluene gave different structures of coke. This can be explained by the ability of hexane t o dissolve resins and precipitate asphaltenes when mixed with bituminous substances. Based on this effect, the lower solubility of coke in toluene after hexane extraction, compared with the original coke, is not surprising. TGA Analysis. Examples of TGA results are shown in Figure 1. These results identify regions where important changes have occurred. For all deposited catalysts the weight loss below 100 "C coincided with that for fresh catalysts and is attributed t o the water desorption. During pyrolysis of the oxidic catalysts (both CoMo and NiMo), two additional regions of weight loss were observed, i.e., at 200 "C and at about 800 "C. During pyrolysis of the sulfided catalysts, the positions of the first and second maximum were unchanged except for the NiMo catalyst deposited with pyrrole. In this case the first maximum was shifted from 200 to about 300 "C. This difference is made evident by comparing the TGA profiles in Figure l a with those in Figure l b . For the sulfided catalysts, the second maximum was shifted to lower temperatures, i.e., for the NiMo and CoMo, the maxima occurred a t about 720 and 750 "C, respectively. As is shown in Figure 2a, using the CoMo catalyst deposited with pyrrole as the example, during TGA oxidation of the oxidic catalysts, two maxima were always observed besides the one below 100 "C which was the same as that for the fresh catalysts, Le., at about 200 and 480 "C. Three main regions were observed during the TGA oxidation of the sulfided deposited catalysts, Le., one at about 200 "C, the second at about 300 "C, and the third a t about 420 "C. The only exception was the NiMo sulfided catalyst deposited with pyrrole. In this case, only two peaks were observed, i.e., one a t about 300 "C and the other at about 400 "C. Pyrolysis of Deposited Catalysts and Alumina. The formation of NH3 during pyrolysis of the deposited CoMo and NiMo catalysts is shown in Figures 3 and 4, respectively. Such results were used to calculate yields and the conversion of the nitrogen in coke t o NH3 and HCN (Table 2). For both catalysts in oxidic form, only one main NH3 peak was observed with a maximum at about 250 "C. A shoulder on this peak indicates a possibility of a second NH3 peak. Two peaks were indeed observed for the sulfided CoMo catalyst with the maximum of the second NH3 peak at about 450 "C. Also, for the sulfided NiMo catalyst, the main peak was shifted to about 450 "C whereas the first peak was diminished and appeared as a shoulder on the onset of the main peak. In every case, pyrrole gave the highest ( 8 ) Corma, A,; Fornes, V.; Rey, F. Zeolites 1992, 13, 56
Regeneration of Spent Hydroprocessing Catalysts
Energy & Fuels, Vol. 9, No. 3, 1995 441
'I
e&o
1
I I
oxidic BtO 60.0
Figure 1. TGA of pyrrole-deposited NiMo catalysts in NP.
yields of NH3, suggesting its lower stability compared with that of pyridine and quinoline. Similar NH3 yields were obtained for catalysts deposited with pyridine and quinoline. HCN was formed only during pyrolysis of the pyrrole-deposited catalysts. The formation of NH3 and HCN during pyrolysis of the y-alumina deposited with pyrrole is shown in Figure 5 . The second NH3 peak, occurring at about 700 "C, and the shift of HCN formation to higher temperatures represent the main difference when compared to deposited catalysts (Figures 3 and 4). The evolution of CO was shifted to a higher temperature by about 150 "C as well. Further, as the results in Table 2 show, the conversion of nitrogen to NH3 was the lowest compared with all pyrrole-deposited catalysts. For the alumina deposited with pyridine and quinoline the maxima of the first NH3 peak occurred at about 500 "C, i.e., at least 100 "C higher than that of the pyrrole-deposited alumina. Similarly, as for the catalysts deposited with
pyridine and quinoline, no HCN was formed for the corresponding deposited aluminas. TPO of Deposited Catalysts and Alumina. The product distribution observed during TPO using 2%0 2 , shown in Table 3, differed markedly from that observed during pyrolysis. Thus, HCN appeared as an important product for every deposited catalyst. Compared with pyrolysis, the appearance of HCN coincided with the decrease of the NH3 yield. Results obtained for the deposited sulfided CoMo catalysts are shown in Figure 6 as an example of the NH3 and HCN profiles. Similar trends were also observed for the sulfided NiMo catalyst. In qualitative terms, a comparison of the TPO of the oxidic catalysts with their pyrolysis yielded a similar observation as that for the sulfided catalysts, i.e., the appearance of HCN and decrease of NH3 yields. However, the former two NH3 peaks observed for both NiMo and CoMo compared with one broad peak observed for corresponding sulfided catalysts. A n example of NH3
Furimsky et al.
442 Energy &Fuels, Vol. 9, No. 3, 1995
2b
/----bm
i
\ "0
1
i
j
sulphided 64.0
!
-L-r-xo 2: 80.0
l a 0
aDCL0
an0
mr.0
Figure 2. TGA of pyrrole-deposited CoMo catalysts in air.
and HCN profiles is shown in Figure 7 for the oxidic NiMo catalyst. For the deposited CoMo catalysts the TPO experiments were also performed using 4% 0 2 . This resulted in a further decrease of NH3 yields and an increase of HCN yields (Table 4) as compared with that obtained in 2%0 2 gas (Table 3). For the deposited oxidic CoMo catalysts, the NH3 yield decrease, caused by increasing 0 2 concentration from 2% to 4%, was much more pronounced than that for the corresponding sulfided catalysts. This is evident from Figure 8 showing the NH3 and HCN formation for the quinoline-deposited catalyst. For both oxidic and sulfided catalysts, increasing 0 2 concentration resulted in an almost complete elimination of low-temperature NH3 peaks. The yield of NH3 during TPO of pyrrole-deposited alumina in 2% 0 2 was the largest compared with all deposited catalysts. At the same time, the yield of HCN was the lowest. Increasing 0 2 concentration from 2 to
4% had little effect on NH3 and HCN yields. The trends
observed for pyridine- and quinoline-deposited alumina differed from those observed for pyrrole-deposited alumina. Thus, during TPO in 2% 02, yields of NH3 were lower than that during pyrolysis. The increase of 0 2 concentration from 2 to 4% resulted in a further decrease of NH3 yields. No HCN was formed during TPO of pyridine- and quinoline-deposited alumina. Similarly, as during pyrolysis, formation of the products was shifted to higher temperatures. The shift was more pronounced for the alumina deposited with pyridine and quinoline than that with pyrrole. Spent Catalysts. The spent CoMo and NiMo catalysts used for hydroprocessing of a distillate and a heavy feed: respectively, were also pyrolyzed. The results in Figure 9a indicate the formation of NH3 and HCN while those in Figure 9b formation of C02, CO, and H2O (9) Kim, C.-S.; Massoth, F. E.; Furimsky, E. Fuel Process. Technol. 1992, 32, 39.
Regeneration of Spent Hydroprocessing Catalysts
-
3.5 3
1
1
oxidic
2.5
2
s
Energy & Fuels, Vol. 9,No. 3, 1995 443 Table 2. Yields of NHs and HCN from Temperature-ProgrammedPyrolysis of Catalyst
P
pyridine quinoline NiMo pyrrole pyridine quinoline
0.5 0
g 2 z
1.5 1
pyrrole pyridine quinoline
0.5 200
600 800 Temperature [‘C]
400
1000
Figure 3. Formation of NH3 during pyrolysis of deposited CoMo catalysts.
0.011 0 0
0.6 0 0
0.23 0.09 0.09
26.3 12.8 13.3
0.010 0 0
0.7 0 0
0.33 0.17 0.13
22.3 15.1 11.4
0.016 0 0
0.7 0 0
0.25 0.05 0.03
14.3 11.1 4.7
0.010 0 0
0.4 0 0
0.15 0.03 0.02
12.4 7.3 4.3
0.015 0 0
1.4 0 0
-
1.2 1
Quinoline Pyridine
O
1-31 HCN
22
!0.4
w’ 0.2 0 0
0
z
15.9 7.2 8.0
a A = wt % of the total catalyst. B = wt % of the total nitrogen in catalyst.
-c 1 ‘5 2 0.8 E 2 0.6
-5 1.5 P
0.18 0.06 0.06
Y-&o3
0
*
B
Sulfided Catalysts CoMo pyrrole pyridine quinoline NiMo pyrrole pyridine quinoline
0
I
A
CoMo
1
s
HCN
Bb Oxidic Catalysts
An
Pyridine
e0 1.5
zz
NH3
guinoline
1 0.5 0
200
400
600
800
1000
Temperature [“C] Figure 5. Formation of NH3 and HCN during pyrolysis of pyrrole-deposited alumina. 4b
i \ 0
200
400
600
Temperature [“C]
sulphided
800
1000
Figure 4. Formation of NH3 during pyrolysis of deposited NiMo catalysts.
during pyrolysis of the spent NiMo catalyst. The main evolution of NH3 occurred in the same temperature range as that of the higher temperature NH3 peak observed for the deposited sulfided catalysts (Figures 3 and 4). A similar observation was also made for the spent CoMo catalyst, except that HCN was not among the products. The TPO of spent catalysts was performed in a n oxidizing gas containing either 2% or 4% 0 2 . A typical
example of NH3 and HCN profiles obtained in 4% 0 2 for spent CoMo and NiMo catalysts are shown in Figure 10, a and b, respectively. The conversions of the nitrogen in coke to NH3 and HCN during pyrolysis and TPO are shown in Table 5 . Compared with pyrolysis, during TPO in 2% 0 2 the HCN yield significantly increased. Increasing 0 2 concentration from 2% t o 4% further increased HCN yield. Relatively large amounts of NH3 were formed during TPO of spent catalysts. This contradicts with a marked decrease in NH3 formation by increasing 0 2 concentration in the oxidizing gas during TPO of deposited catalysts. Potential for NzO Formation. For selected catalysts, experiments were performed to verify the formation of N2O during both pyrolysis and TPO. There was no clear evidence supporting the N2O formation during pyrolysis of the deposited oxidic catalysts as well as the spent NiMo and CoMo catalysts. However, for the spent NiMo catalyst, an N2O peak with a maximum of about 4 and 10 ppm appeared during TPO in 2%and 4% 02, respectively, between 600 and 800 “C. It was determined in a separate experiment that the NO formation was almost complete before this temperature region was attained. As the results in Figure 11show, rather large amounts of N2O were formed during the TPO of pyrrole-
Furimsky et al.
444 Energy & Fuels, Vol. 9, No. 3, 1995
0.5
1.4
I
-
I
69
4
&
_0.4
'
E
-i0.3 4 3
NH3
A
I
Pyridine d
Quinoline
Pyrrole
0.8
Pyridine
I
Pyrrole
0 0
200
600 800 Temperature ["C]
1000
400
0
Figure 6. Formation of NH3 and HCN during TPO of deposited sulfided CoMo catalysts in 2%
02.
Table 3. Yield of N H 3 and HCN from TPO in 2 vol % + Balance NZ
NH3
HCN
B Oxidic Catalysts
A CoMo pyrrole pyridine quinoline NiMo pyrrole pyridine quinoline
A
400 600 800 Temperature ["C]
1000
deposited oxidic NiMo catalysts in 2% 0 2 . Table 4. Yields of N H 3 and HCN during TPO of Deposited CoMo Catalysts in 4 vol % 0 2
NH3
HCN
B
A
B
0.3 0.5 0.9
0.097 0.022 0.025
7.5 1.7 2.1
A
Oxidic
0.062 0.025 0.019
7.6 3.0 2.6
0.066 0.017 0.022
5.2 1.3 1.8
0.116 0.059 0.048
13.3 8.4 7.2
0.050 0.018 0.019
3.5 1.6 1.8
0.115
0.036 0.024 0.017
2.4 2.1 1.5
0.089 0.070
4.9 5.0 3.9
0.078 0.013 0.026
4.5 2.9 4.0
0.121 0.028 0.044
4.4 3.9 4.3
0.178 0.017 0.016
25.7 4.1 3.5
0.004
0.4 0 0
Y--%o3
pyrrole pyridine quinoline
200
Figure 7. Formation of NH3 and HCN during TPO of
B
Sulfided Catalysts CoMo pyrrole pyridine quinoline NiMo pyrrole pyridine quinoline
"-
i i ,
0 0
deposited CoMo catalyst. Also, in this case the evolution of N2O began later than that of NO. As the results in Table 6 show, increasing 0 2 concentration from 2 to 4% resulted in a significant increase in N2O yield. For sulfided pyrrole-deposited CoMo and NiMo catalysts, N20 yields were lower than that for the corresponding oxidic catalysts. During TPO of pyridine- and quinolinedeposited catalysts, the NzO concentration never exceeded 10 ppm. Overall Mass Balance of Nitrogen. The summary of all results obtained for deposited catalysts shows that NH3, HCN, NO, and NzO account for less than 30% of the nitrogen in catalysts. The spent NiMo catalyst and sulfided quinoline-deposited CoMo catalyst were pyrolyzed to verify the formation of N2. The results in
deposit Pyrrole pyridine quinoline
0.002 0.004 0.007
Sulfided deposit Pyrrole pyridine quinoline a1umina pyrrole pyridine quinoline
0.021 0.021 0.023
1.4 1.9 2.0
0.148 0.109 0.108
6.3 6.1 6.0
0.184 0.008 0.011
26.5 1.9 2.4
0.004 0 0
0.4 0 0
Figure 12 indeed confirm the N2 formation, i.e., 44.5 and 24.1% for NiMo and CoMo catalysts, respectively. In addition to this, analysis of the NiMo catalyst after the pyrolysis tests indicated the presence of residual nitrogen in spite of rather high temperature and a long soak time. Part of the nitrogen was also removed as a tar. In the present work, no attention was paid t o this material.
Discussion The shift of the NH3 peak during pyrolysis of sulfided catalysts to higher temperatures, as shown in Figures 3 and 4,can explain the shift of the weight loss to higher temperature during TGA of sulfided catalyst, as shown in Figure lb. Different profiles of NH3 evolution shown in Figures 3 and 4 suggest that for the oxidic catalysts the interaction of hetero rings with CoMo catalyst differs from that with NiMo catalyst. The sulfidation had different effects on the interaction, i.e., for the latter, conversion of nitrogen t o NH3 decreased whereas that
Regeneration of Spent Hydroprocessing Catalysts
Energy & Fuels, Vol. 9, No. 3, 1995 445
1 , 1.2 f l a 0.8
I
T
s
I
0.6 a
g!
e 0.4
c
sulphided
w
0.2
0.2
!
0
i
Sb-
oxidic
1.2
I
r, 1
lob
II
I
HCN
F
, HCN
0.6
f
0.4
4
1:m 'E a 0.8 1 a
fi
HCN IT\ e
I
f 0.6
,$0.4 1 0.2 -
0 200
0
400 600 800 Temperature ["C]
0
1000
Figure 8. Formation of NH3 and HCN during TPO of quinoline-deposited CoMo catalysts in 4%0 2 .
U,"" x~
I 1 1 1
" "
200
0
400 600 800 Temperature ["C]
1000
Figure 10. Distribution of NH3 and HCN during TPO of spent CoMo and NiMo catalysts in 4% 0 2 .
1
I
Table 5. Effect of 0 2 Concentration on Conversion of Nitrogen to NHs and HCN for Spent Hydroprocessing Catalysts
-.- 0.8 E
%
f
concentration, vol %
0.6
I
9 E
0.4
$ w
I I
0.2 0
-
g
w-
140 120 100
co
H20
~
5d
80 60 2! 40
CoMo NH3 HCN NiMon NH3 HCN N2 NiMob NH3 HCN
0
2
4
9.9 0
11.9 15.0
11.4 16.4
14.4 0.5 44.5
11.5 7.8 -
8.7 8.4 -
7.8 1.9
9.8 9.2
7.2 11.0
Extracted by hexane followed by toluene. Directly extracted by toluene. 2.5
E
CoMo
20 0 0
200
400
600
Temperature ["C]
800
1000
Figure 9. Distribution of products during pyrolysis of spent NiMo catalyst.
for CoMo catalyst increased. For pyrrole-deposited CoMo catalyst, the same trends were also observed for HCN. The appearance of two NH3 peaks during pyrolysis of sulfided catalysts suggests that there are at least two different sources of NH3. A part of NH3 could have been formed by hydrogenation of HCN or that of a precursor leading to HCN formation. The formation of NH3 by HCN hydrogenation was indeed confirmed during pyrolysis of coaL6J0 In the case of the catalysts, there are at least two sources of hydrogen, i.e., hydrogen in coke and hydrogen associated with catalyst surface.
200
300
500 600 Temperature ["C]
400
700
800
Figure 11. N2O formation during TPO of pyrrole deposited oxidic CoMo and sulfided NiMo catalysts in 4% 0 2 .
It is believed that for sulfided catalysts, a part of hydrogen is in the form of -SH groups.l This part of hydrogen may be at least partly responsible for the
Furimsky et al.
446 Energy & Fuels, Vol. 9, No. 3, 1995
4 0.5 0
0
200
400 600 800 Temperature [“C]
1000
Figure 12. Formation of Nz during pyrolysis of spent NiMo catalyst and quinoline-deposited CoMo catalyst. Table 6. Conversion of Nitrogen to NzO and NO (%) catalyst CoMo oxided pyrrole
0 2
in gas
NzO
NO
4
0 0.8 6.2 40.5
0 nd 1.7 1.2
4
1.2
0.9
4
1.6
nd
0 2
0 40.5 40.5
0
2 4
pyridine CoMo sulfided pyrrole NiMo sulfided pyrrole NiMo spent
4
0
nd 21.7
appearance of the second NH3 peak at higher temperatures. Then, the lower temperature NH3 could have been generated by HCN hydrogenation using coke’s hydrogen. Also, participation of some labile surface -OH groups in the hydrogen transfer cannot be ruled out. It appears that this source of hydrogen became less important for sulfided catalysts. This is evident especially for NiMo catalysts. The non-HCN route of NH3 formation is also possible, especially for pyrroledeposited catalysts. In this case, the NH species would be an obvious precursor. It is more difficult to explain a direct non-HCN formation of NH3 from pyridine- and quinoline-deposited catalysts. The shift of the NH3 and HCN peaks during pyrolysis of pyrrole-deposited alumina (Figure 5) to higher temperatures indicates a stronger interaction of the rings with the surface than that on the corresponding deposited catalysts. In other words, the active metals when added to the alumina support modify interaction of the rings with the latter. For all deposited catalysts, the appearance of HCN, accompanied by the decrease of NH3 during the oxidation in 2% 0 2 was a common observation. This would confirm that a t least a part of NH3 was formed by hydrogenation of either HCN or its precursor. Thus, in the presence of 0 2 the availability of hydrogen is significantly reduced. 0 2 may have also reacted with intermediates which lead to the formation of HCN and NH3. A surface oxidation reaction in which oxygen attacks selective bonds leading to the formation of HCN is also possible.ll During TPO, this reaction may successfully compete with the pyrolysis reaction (thermal or surface aided), which leads t o the formation of an NH3 precursor. The decrease in NH3 concentration (10) Chen, J. C.; Niksa, S. Energy Fuels 1992,6, 254. (11)Massoth, F. E., private communication.
was more pronounced on sulfided catalysts when pyrolysis and TPO in 2% 0 2 are compared. Thus, both low- and high-temperature NH3 peaks observed during pyrolysis were significantly decreased during TPO in 2% 0 2 . This suggests either a more extensive depletion of active hydrogen from the surface or an improved oxidation of the organic matter yielding HCN. It is well established that during oxidation, most of catalyst’s sulfur as part of MoS2 and the surface hydrogen which may be associated with it are removed at about 300 “C.l This may be partly responsible for lowering NH3 yields. However, for CoMo catalysts, a further 0 2 concentration increase from 2 to 4% had a much more pronounced effect on its oxidic form (Tables 3 and 4). As the results in Figure 7 show, two different sources of NH3 are still evident for the oxidic deposited catalysts. But again, these peaks are much smaller when compared with pyrolysis (Figure 4). The yields of NH3 during TPO of pyrrole-deposited alumina in 2 and 4% 0 2 were the largest compared with other deposited catalysts and aluminas. This supports a participation of the surface H-containing groups, i.e., most likely some active -OH groups. Thus, it is unlikely that hydrogen in coke could cause such a significant increase in NH3 yield. In fact, the results for all deposited catalysts show that the availability of such hydrogen is affected in the presence of 0 2 . Participation of -OH groups may also be supported by the absence of HCN among products from the TPO of pyridine- and quinoline-deposited aluminas compared with corresponding deposited catalysts. It appears that most of the NH3 formed on the alumina does not originate from HCN or its precursor. It is evident that both pyrolysis and oxidation of pyridine and quinoline on alumina differ from that of pyrrole. Most likely, a different mode of adsorption at the surface may be responsible for this difference. Large yields of NH3 were formed during both pyrolysis and TPO of the spent catalysts. In two cases, the yield of NH3 even increased in 2% 0 2 compared with pyrolysis. For the spent catalysts, a significant increase in the yield of HCN during TPO should be noted. In fact, large amounts of both NH3 and HCN were always formed during TPO of the all spent catalysts tested. Compared with pyrolysis, combined yield of NH3 and HCN during TPO of the spent CoMo catalyst almost tripled. This would suggest an 0 2 aided decomposition of N-containing structures. It is believed that a t least for the spent CoMo catalyst, most of the NH3 was originated from a non-HCN source. Thus, the increase in 0 2 concentration from 2 to 4% had only a minor effect on NH3 yield (Table 5). In this case, amino groups would be a natural source of NH3. Considering the hydroprocessing conditions t o which the catalysts are usually subjected, the presence of such groups in the coke is highly probable. Amino groups may contribute to NH3 formation also during pyrolysis and TPO of spent NiMo catalysts. However, a more pronounced decrease of NH3 yield by increasing 0 2 concentration from 2 to 4% supports a contribution from other sources, i.e., hydrogenation of HCN or its precursor to NH3. Further, a much thicker coke layer on the spent NiMo catalyst (e.g., about 20 wt % of coke compared with less than 10 wt % on the spent CoMo catalyst) prolongs a contact of HCN and its precursors with the coke. Also, a part of
Regeneration of Spent Hydroprocessing Catalysts
the 0 2 will be preferentially reacting with the outer surface of the coke layer. In other words, a t least for a part of TPO, the availability of hydrogen in the interior of the coke layer may not be affected by 0 2 . These facts would increase probability of NH3 formation and decrease that of HCN especially if the surface oxidation recation is responsible for the formation of the latter. It is unlikely that N2O was formed in gas-gas oxidation of either HCN or NH3 because of very low concentrations of these species. Also, it was shown that the oxidation of NH3 and HCN to N20, when injected into a flame, occurs above 800 OC.12 Therefore, a gassolid oxidation of nitrogen is a probable route. A direct N20 formation requires the presence of two nitrogen in a proximity. It is generally believed that the nitrogen hetero rings are attached t o the surface via an N heter0atom.l If so, the surface nitrogen concentration required for N2O formation was attained for pyrroledeposited catalysts. N2O formation by reduction of NO by the remaining coke may also occur.13 The large amount of coke on the spent NiMo catalyst would favour this reaction. Nevertheless, relatively large yield of NO and very small yield of N20 during TPO of the spent NiMo catalyst suggest that for the spent catalysts this source of NzO is unimportant. The results suggest that at the highest temperature usually applied during regeneration of hydroprocessing catalysts (e.g., less than 500 "C), a portion of nitrogen may remain in the catalyst. Little is known about the form of such nitrogen, though a nitride type is assumed to be present. Also, an association of nitrogen with the active components of regenerated catalysts affecting recovery of their activity cannot be ruled out completely. (12) Kramlich, J. C.; Cole, J. A.; McCarthy, J. M.; Lanier, W. S.; McSorley, J. A. Combust. Flame 1989,77,375. (13)De Soete, G. Rev. Znst. Fr. Pet. 1993,48, 413.
Energy & Fuels, Vol. 9, No. 3, 1995 447
This issue may deserve some attention. Thus, an isothermal burnoff below 500 "C may remove most of nitrogen if a sufficient contact time is used. The studies on kinetics of regeneration so far have focused mostly on carbon, hydrogen, and sulfur with little attention being paid to nitrogen.
Conclusions The NH3, HCN, and NO are the main N-containing compounds formed under 0 2 deficient conditions such as those applied during regeneration of spent hydroprocessing catalysts, whereas only very small amount of N2O was formed. Part of the coke's nitrogen may survive regeneration and can be removed as N2 at much higher temperatures than those employed during regeneration. A significant increase in HCN yields during TPO of the catalysts suggests that HCN may be a precursor t o NH3 formation. However, non-HCN routes to NH3 may also exist especially for the spent catalysts and alumina. Catalyst sulfidation influenced adsorption of nitrogen hetero ring as well as yields and distribution of products during pyrolysis and TPO. b o l e waa always the most reactive compared with pyridine and quinoline. The appearance of HCN and significant decrease in NH3 yield during TPO suggest that HCN may be a precursor to the latter. The required hydrogen may be provided either by the organic deposit or catalyst surface. During TPO, the HCN may arise from the oxygen-aided oxidation of selective bonds. Active metals when added to the alumina support seem to diminish the interaction of nitrogen hetero rings with the latter. They also influence yields and distribution of HCN and NH3 during both pyrolysis and TPO. EF940227D
